The hypodermic syringe is perhaps the most common hypodermic injector. It consists of a barrel fitted with a plunger and a hollow needle. Mechanically driven syringes are employed to infuse drugs into patients continuously over relatively long periods.
U.S. Pat. Nos. 3,814,097, 3,964,482, and 4,159,720 disclose devices for administering drugs percutaneously on a continuous basis that include integral components that penetrate the skin. The latter employs wicks or monofilaments that are installed in the subcutaneous flesh and are connected to a drug reservoir attached to the skin surface. The drug is transported from the reservoir into the flesh by capillary action. The other two patents use hollow skin piercing projections that extend from the undersides of the respective devices.
A variety of osmotically driven fluid dispensers or injectors are either available commercially or described in the literature. These dispensers operate by imbibing water through a semipermeable membrane to displace fluid--typically a drug or other active agent--from the dispenser. Rose and Nelson, Austral. J. exp. Biol., 1955, 33, pp 415-420 describes one such injector. Its main components are a glass ampoule, two latex bags, and a semipermeable cellophane membrane mounted in a ring clamp. The ampoule has a nipple at one of its ends and a splayed mouth at the other end. One of the latex bags is partly collapsed, contains a saturated solution of Congo Red, and is tied over the clamp at a point below the bag mouth. This bag is placed in the ampoule with its edge tied over the splay of the ampoule mouth. The second bag is filled with water and its mouth is also tied over the splay of the ampoule mouth. Drug is contained in the space within the ampoule not occupied by the ring-first bag assembly. In operation, water is imbibed osmotically from the second bag through the cellophane member into the first bag causing the first bag to inflate. This inflation displaces drug from the ampoule via the nipple.
U.S. Pat. No. 3,604,417 describes another such injector which consists of three rigid chambers arranged in tandem. The first chamber contains the drug and has an outlet. The second contains a solution of an osmotically effective solute and is separated from the first by a piston. The third contains water and is separated from the second by a semipermeable membrane. In operation, water from the third chamber is imbibed through the semipermeable membrane into the second chamber due to an osmotic imbalance between the contents of the two chambers. The influx of water into the second chamber drives the piston that separates the second and first chambers into the first chamber, thereby displacing drug therefrom via the outlet.
U.S. Pat. No. 3,987,790 describes another type of osmotically driven dispenser. Its basic components are an inner flexible bag that contains the fluid to be dispensed, an intermediate layer of an osmotically effective composition that encapsulates the inner bag, an outer rigid semipermeable coating that encapsulates both the inner bag and intermediate layer, and an outlet/filling port that communicates with the interior of the inner bag. In operation the inner bag is charged with the fluid to be dispensed via the filling/discharge port and the dispenser is placed in an aqueous environment, such as a body cavity or within body tissue. Water is imbibed from the environment by the osmotically effective composition through the semipermeable coating into the space between the coating and the inner bag. Since the bag is flexible and the coating is rigid, the imbibed water squeezes the bag inwardly thereby displacing fluid from the bag via the discharge/filling port.
A principal object of the present invention is to provide a self-driven hypodermic injector that will inject a liquid pharmaceutical formulation into a patient at a constant controlled rate.
Another object of the present invention is to provide such an injector whose component parts align axially and thus may be assembled easily.
Yet another object of the present invention is to provide such an injector that includes means for supplementing the constant rate injection with one or more pulse injections of the pharmaceutical formulation.